Who discovered the first planet and how did they know it was a planet?

Duncan Steel, a space researcher, author and broadcaster who now lives in Wellington, responded.

The word ‘planet’ is derived from an ancient Greek word meaning ‘wanderer’.
This is because these objects were observed to wander around, as compared
to the background of distant stars which move across the sky in step.

The ancients recognised seven such ‘planets’, the list including the Sun and the Moon.
The other five planets that are easily visible to the naked eye and so have been
known since antiquity are Venus, Mars, Mercury, Jupiter and Saturn. It is
because of this that we have seven days in a week. The naming of Sunday
and Monday is obvious, as is Saturday, but all are named for Nordic gods:
Tuesday for Mars (Tiw), Wednesday for Mercury (Woden), Thursday for
Jupiter (Thor), and Friday for Venus (Frige). The names are more obvious
in other languages (look up words for each day in French, Italian and Spanish).

The first new planet to be discovered was Uranus, spotted by William Herschel
from Bath in England in 1781. His house there is now a museum celebrating
his astronomical work. In fact Herschel initially thought it was a comet.
Another interesting point is that it can – just – be seen with the naked eye
at some times, from a very dark site, if your eyesight is really good.

Over the next several decades Uranus was observed to ‘wobble’ slightly
in its orbit, leading to the prediction of another planet still further from the
Sun, its gravity tugging at Uranus. As a result Neptune was discovered,
at the Berlin Observatory, in 1846. About thirty years ago it was realised
that Galileo had observed Neptune with his newly-invented telescope
early in the 17th century, but thought it was a star.

Pluto was discovered in 1930, but professional astronomers have in
recent years downgraded it to the status of a ‘dwarf planet’ because
several other similar-sized objects have been spotted out beyond Neptune
using very large telescopes.

Another class of ‘planet’ consists of the ‘minor planets’, most of which
orbit the Sun between Mars and Jupiter. A synonym for ‘minor planet’ is
‘asteroid’, meaning ‘star-like’, a word coined by William Herschel soon
after the first such object was discovered on 1st January 1801, from
Palermo in Sicily.

There are billions of minor planets/asteroids. One (4713 Steel) is named for
me, whilst I have discovered another dozen. My sons have two of those
named for them: look up ‘5263’ Arrius and ‘6828 Elbsteel’ on Wikipedia.

Since 1995 some hundreds of large planets have been discovered in
orbits around other stars, provoking speculation as to whether there
might be life elsewhere in our galaxy.